1. Transition elements Cobalt
Electron configuration
[Ar]4s3d7
Physical properties of cobalt
Cobalt is a silver-white lustrous metal, slightly harder than iron. It is malleable and ductile.
It is more magnetic than all other metals except iron.
It has a density equal to 8.9. It melts at 14950C and boils at 29000C.
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2. Transition elements Cobalt
Properties as a transition element
Forms colored compounds; for instance Co2+ is pink
Has variable oxidation states; +2, +3,
Forms complexes, e.g. CoCl42-.
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3. Transition elements Cobalt
Action of air:
Dry air has no action on cobalt but the metal reacts with O2 to form Co3O4 when red hot.
3Co(s) + 2O2(g) → Co3O4(s)
Action of water: Cobalt does not react with cold water. However, it decomposes steam to hydrogen. Co(s) + H2O(steam) → CoO(s) + H2(g)
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4. Transition elements Cobalt
Action of acids
Dilute Hydrochloric acid and sulphuric acid form salts and hydrogen with cobalt.
Co(s) + 2H+(g) → Co2+(aq) + H2(g)
Dilute nitric acid dissolves the metal and renders it passive due to formation of a protecting thin layer of metal oxide on the surface of the metal.
Action of alkalis:
Even concentrated alkali has no action on the metal. The metal is dissolved in fused caustic potash at 5500C.
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5. Transition elements Cobalt
Action of Carbon monoxide:
Finely divided Cobalt combines with carbon monoxide at 1500C and under 30 atmospheres to form orange-red Co2(CO)8.
Action of ammonia:
Co4N2 is formed.
4Co(s) + 2NH3(g) → Co4N2(s) + 3H2(g)
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6. Transition elements Cobalt
Action with other elements:
Cobalt readily combines with H2, S, P and B but N2 has no action on it.
Co(s) + H2(g) → CoH2(s)
Co(s) + S(s) → CoS(s)
3Co(s) + 2P(s) → Co3P2(s)
Uses of cobalt
Cobalt is used for electroplating.
For preparation of alloys; e.g. cobalt steel for making permanent magnets, stellite (Co, Cr and W (tungsten)) for making surgical instruments, and Co-chrome (Co, Cr and Fe) for making resistance wires.
Cobalt oxides and silicates are used for coloring glass, pottery and glazes.
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7. Transition elements Cobalt
Action with other elements:
Cobalt readily combines with H2, S, P and B but N2 has no action on it.
Co(s) + H2(g) → CoH2(s)
Co(s) + S(s) → CoS(s)
3Co(s) + 2P(s) → Co3P2(s)
Uses of cobalt Cobalt is used for electroplating. For preparation of alloys; e.g. cobalt steel for making permanent magnets, stellite (Co, Cr and W (tungsten)) for making surgical instruments, and Co-chrome (Co, Cr and Fe) for making resistance wires. Cobalt oxides and silicates are used for coloring glass, pottery and glazes
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8. Transition elements Cobalt
Qualitative analysis of cobalt II salts
Common salt: Cobalt sulphate: CoSO4
With dilute sodium hydroxide solution of ammonium solution
Observation
Blue precipitate insoluble in excess turns brown on standing
Confirmatory test
(a) Forms deep blue solution with concentrated hydrochloric acid.
Co(H2O)62+ (aq) + 4Cl- (aq) →
CoCl42- (aq) + 6H2O (l)
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9. Transition elements Cobalt
Confirmatory test
(b) With Concentrated ammonia forms a blue precipitate soluble in excess to give pale yellow solution; the solution turns brown on standing.
[Co(H2O)6]2+ (aq) + 2OH- (aq) → Co(OH)2 (s)
pink blue ppt
Co(OH)2 (s) + 6NH3 (aq) s → [Co(NH3)6]2+(aq)
red-brown
[Co(NH3)6]2+(aq) →Co(NH3)6]3+(aq)
red-brown yellow solution
Trial 23 (2004/1/15)
(a) Write equations for the reactions which take place and in each case, state what would be observed when;
(i) Concentrated ammonia is added drop wise to an aqueous solution of cobalt (II) sulphate and when the resultant solution is allowed to stand in air. (4 marks)
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